Unwrap CompletionException when dealing with CompletionStage in REST …

…server

Fixes: quarkusio#41996

extensions/resteasy-reactive/rest/deployment/src/test/java/io/quarkus/resteasy/reactive/server/test/MutinyAsCompletionStageTest.java

@@ -0,0 +1,68 @@
+package io.quarkus.resteasy.reactive.server.test;
+
+import static org.hamcrest.Matchers.equalTo;
+
+import java.util.concurrent.CompletionStage;
+import java.util.function.Supplier;
+
+import jakarta.ws.rs.BadRequestException;
+import jakarta.ws.rs.GET;
+import jakarta.ws.rs.Path;
+import jakarta.ws.rs.Produces;
+import jakarta.ws.rs.core.MediaType;
+
+import org.jboss.shrinkwrap.api.ShrinkWrap;
+import org.jboss.shrinkwrap.api.spec.JavaArchive;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+import io.quarkus.test.QuarkusUnitTest;
+import io.restassured.RestAssured;
+import io.smallrye.mutiny.Uni;
+
+public class MutinyAsCompletionStageTest {
+
+    @RegisterExtension
+    static QuarkusUnitTest test = new QuarkusUnitTest()
+            .setArchiveProducer(new Supplier<>() {
+                @Override
+                public JavaArchive get() {
+                    return ShrinkWrap.create(JavaArchive.class)
+                            .addClass(TestResource.class);
+                }
+            });
+
+    @Test
+    public void testOk() {
+        RestAssured.get("/test/ok")
+                .then()
+                .statusCode(200)
+                .body(equalTo("test"));
+    }
+
+    @Test
+    public void testError() {
+        RestAssured.get("/test/error")
+                .then()
+                .statusCode(400);
+    }
+
+    @Path("test")
+    public static class TestResource {
+
+        @GET
+        @Produces(MediaType.TEXT_PLAIN)
+        @Path("/ok")
+        public CompletionStage<String> ok() {
+            return Uni.createFrom().item("test").subscribeAsCompletionStage();
+        }
+
+        @GET
+        @Produces(MediaType.TEXT_PLAIN)
+        @Path("/error")
+        public CompletionStage<String> error() {
+            return Uni.createFrom().<String> failure(new BadRequestException()).subscribeAsCompletionStage();
+        }
+    }
+
+}

independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/handlers/CompletionStageResponseHandler.java

@@ -1,6 +1,7 @@
 package org.jboss.resteasy.reactive.server.handlers;
 
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionException;
 import java.util.concurrent.CompletionStage;
 import java.util.concurrent.atomic.AtomicBoolean;
 
@@ -28,7 +29,11 @@ public void handle(ResteasyReactiveRequestContext requestContext) throws Excepti
                     }
                 } else {
                     if (t != null) {
-                        requestContext.handleException(t, true);
+                        if (t instanceof CompletionException ce) {
+                            requestContext.handleException(ce.getCause(), true);
+                        } else {
+                            requestContext.handleException(t, true);
+                        }
                     } else {
                         requestContext.setResult(v);
                     }